1. Field of the Invention
The present invention relates to a fuel cell and a bonding structure of a separator for the fuel cell. More specifically, the invention is directed to an improved bonding structure of a separator that is provided in a unit cell of a fuel cell.
2. Description of the Related Art
A fuel cell is typically fabricated by stacking a plurality of unit cells. In such a fuel cell, e.g., in a solid polymer type fuel cell, each of the unit cells includes a membrane electrode assembly (“MEA”), having an electrolyte membrane and a pair of electrodes respectively arranged on both surfaces thereof, and a pair of separators for sandwiching the membrane electrode assembly between the separators. A number of such unit cells are stacked on one another to provide a fuel cell of stacked structure. A metal separator is used as the separator. Certain types of fuel cells include a resin frame that is inserted between the metal separator and the membrane electrode assembly. In this type of fuel cell, the metal separator is bonded to the resin frame by an adhesive agent. Furthermore, it is typical that sealing is provided by interposing a gasket between unit cells (see Japanese Patent Application Publication No. JP-A-2003-77499, for instance)
In the fuel cell referred to above, it is often the case that an internal pressure generated in an operating process of the fuel cell applies a force to the adhesive agent. For this reason, it is desirable that the adhesive agent is provided below the gasket so that it still can effectively perform the sealing function even under such circumstances. This ensures that the force attributable to the internal pressure is counterbalanced by the reaction force of the gasket, thereby inhibiting any peel-off of the adhesive agent.
However, the prior art fuel cell suffers from a problem in that, in the process of operating the fuel cell for instance, the adhesive layer is subject to elastic or plastic deformation due to the reaction force of the gasket, thereby causing deformation to the metal separator.
In other words, when the fuel cell is operated at a temperature within a range of, e.g., from sub-zero to 90° C., misalignment between the metal separator and the resin frame often occurs due to the difference in their linear expansion coefficients (thermal expansion coefficients). Thus, a thick adhesive layer should be formed to avoid an occurrence of such misalignment. In the meantime, a high level of sealing performance required of the gasket interposed between the individual unit cells makes the gasket exert a reaction force (e.g., elasticity-attributable repulsive force) in itself. The adhesive layer is deformed by the reaction force of the gasket, which in turn often causes deformation to the metal separator (e.g., stainless steel separator) that has a reduced thickness and a low bending strength. Another problem is that this may degrade or deteriorate sealability in between the gasket and the separator.